1. Field of the Invention
This invention relates generally to data recording channel signal detectors and, more specifically, to a digital system for detecting waveform peaks or threshold-crossings in an asynchronously-sampled data signal.
2. Discussion of the Related Art
In the data storage arts, binary data is encoded and stored on optical or magnetic media as a series of optical or magnetic threshold-crossings. Retrieval of stored data requires a detecting and decoding system in the recording channel to reconstruct the original binary data and synchronous clock from the self-clocking analog signal waveform created by the magnetic or optical transducer adjacent to the storage medium. Practitioners in the art have proposed many solutions to the problems associated with extracting synchronous clock and data signals from a self-clocking data signal. These problems include discrimination between actual threshold-crossings and mere noise pulses and precise reconstruction of the synchronous data clock signal for decoding the phase-encoded data.
Analog pulse detectors known in the art suffer from the usual disadvantages of analog electronic apparatus. They are expensive, bulky and subject to calibration drift over time. Moreover, analog pulse detectors are generally suited to a narrow predetermined range of channel data rates, imposing severe channel data rate restrictions on storage media data retrieval systems. Digital implementations of data pulse or threshold-crossing phase detectors known in the art usually rely on discrete-signal embodiments of the well-known analog detection techniques. For instance, the analog signal waveform is first sampled and digitized using well-known phase-locked loop (PLL) techniques. These samples are then processed digitally to remove unwanted frequency components and to reconstruct the synchronous clock and data.
Such a discrete-signal equivalent of a phase-locked loop circuit is disclosed by Antonia C. Van Rens, et al in U.S. Pat. No. 4,912,729. Van Rens et al disclose a digital method for detecting data pulse transitions in an asynchronously-sampled signal waveform, but their technique is restricted to controlling estimated signal threshold-crossing phase errors using a discrete-time oscillator under the control of a digital sequential filter to hold the phase error to substantially zero. The Van Rens et al digital PLL method requires asynchronous sampling rates substantially higher than the expected channel data rate. Also, if the data signal threshold-crossings temporarily drop out, their discrete-time oscillator drifts off-clock very rapidly, creating relocking delays when the data signal resumes. This also limits the variety of Run-Length Limiting (RLL) codes suitable for their device.
There is a clearly felt need in the art for a fully digital implementation of a channel waveform threshold-crossing phase detector that can accurately detect self-clocking data pulses in a recording channel data signal over a wide range of data rates. The crucial need is for accurate synchronous data detection at moderate asynchronous sampling rates because high-speed sampling techniques are disadvantageously expensive. Although decimation and interpolation techniques are available for translating sampling rates from one level to another, these techniques alone do not facilitate detection at unrestricted data rates in a data recording channel. Moreover, such sampling rate translation overhead militates against rapid, efficient decoding of phase-encoded data signals.
Digital computer programs are known in the art for simulating the functions of analog recording channels but these techniques require high sampling rates, making implementation in real-time hardware expensive and difficult as discussed above. Many of the difficulties known for digital implementation can be overcome by reducing the asynchronous analog signal sampling rate. A practical method for accurate digital phase detection at low sampling rates has not been known until now. These and other unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.